[python,parallel] improve compatibility of Dune::Communication<...> and mpi4py

• add MPI_Comm wrapper
• add conversion from mpi4py.MPI.Comm to MPI_Comm wrapper
• allow to construct dune.common.Communication from mpi4py.MPI.Comm
• additional tests
• possibly move to a submodule parallel (discussion postponed)

dune/common/parallel/mpicommunication.hh

@@ -129,6 +129,11 @@ namespace Dune
       : Communication(MPI_COMM_SELF)
     {}
 
+    //! Converting constructor for No_Comm that is interpreted as MPI_COMM_SELF
+    Communication (const No_Comm&)
+      : Communication(MPI_COMM_SELF)
+    {}
+
     //! @copydoc Communication::rank
     int rank () const
     {